The present invention relates to a positioning servomechanism. More particularly, the present invention relates to a positioning servomechanism operating between endstops, where a high speed collision with the endstops would damage the servomechanism itself or the equipment being positioned. Still more particularly, the present invention relates to the style of servomechanism which is used to position a radially movable transducer head over data storage tracks on a disk. In greatest particularity, the present invention relates to a disk storage system employing a head positioning servomechanism where provision is made to prevent damage in the event a fault condition occurs.
The use of servomechanisms to position radially movable read/write heads over data storage tracks on disks is a well known art. The head is moved between endstops representing the least and greatest radii of its range. The read/write head is fragile, and is mounted on flimsy flexures. If the head collides violently with either of its endstop positions, it is likely to be seriously damaged. Also, the head flexures, or the servomechanism actuator itself, may also be damaged. In addition, there is usually provided a head unloading ramp, up which the head is parked when removed from the surface of the disk. Collision of the head with this ramp at high speed also places the head in extreme risk of sustaining damage.
The trend towards higher performance disk drives has resulted in the abandoning of relatively safe but slow stepping motor head positioning servomechanisms in favor of faster actuators, where a force positions the head under the control of positional feedback information and externally applied demand and control signals. Should any fault develop in the servomechanism or the equipment providing demand and control signals, then there is a risk that the actuator (and thus the head) will be accelerated uncontrollably towards one or other of its endstops, thereby destroying the head. Because of the relatively high cost of heads, such destruction can be very costly. Also, the replacement of a head is a delicate and time-consuming operation.
One approach to the solution of the above described problem is disclosed in U.S. Pat. No. 4,375,611. In this approach, an inhibiting circuit is provided for monitoring the servo output voltage applied to the head actuator. The inhibiting circuit operates to disconnect the servo output from the actuator if it detects that the servo output voltage remains at a predetermined maximum value for a time sufficient to cause the head to reach a velocity which would damage the head if it impacts with an endstop. The difficulty with this solution to the head damage problem is that the inhibiting circuit, which employs active elements such as transistors, is itself subject to fault. As a result, although the number of damaging head crashes which occur is reduced by the use of such an inhibiting circuit, the number of damaging head crashes which still occur is undesirably high, particularly in view of the high cost of a head and the operating time which is lost until the head can be replaced.